1. Field of the Invention
The prevent invention relates to a thermal transfer image-receiving sheet and more particularly to a thermal transfer image-receiving sheet for use in a thermal transfer recording system wherein a sublimable dye is used as a colorant.
2. Background Art
Various thermal transfer recording systems are known in the art, and one of them is a dye sublimation transfer recording system in which a sublimable dye as a colorant is transferred from a thermal transfer sheet to an image-receiving sheet by means of a thermal head capable of generating heat in response to recording signals, thereby forming an image. In this recording system, since a dye is used as colorant and the gradation of the density is possible, a very sharp image can be formed and, at the same time, the color reproduction and tone reproduction of half tones are excellent, making it possible to form an image go having a quality comparable to that formed by silver salt photography.
By virtue of the above excellent performance and the development of various hardwares and softwares associated with multi-media, the dye sublimation transfer recording system has rapidly increased the market in a full-color hard copy system for computer graphics, static images through satellite communication, digital images represented by CD-ROM, and analog images such as video.
Specific applications of the image-receiving sheet in the dye sublimation transfer recording system are various, and representative examples thereof include proof printing, output of an image, output of a design, such as CAD/CAM, output applications for various medical instruments for analysis, such as CT scan, output applications for measuring equipment, alternatives for instant photography, output of photograph of a face to identification (ID) cards, credit cards, and other cards, and applications in composite photographs and pictures for keepsake in amusement facilities, such as pleasure grounds, museums, aquariums, and the like.
The thermal transfer image-receiving sheet for dye sublimation transfer used in the above various applications (hereinafter referred to simply as "thermal transfer image-receiving sheet" or "image-receiving sheet") generally comprises a substrate (referred to also as a "support") and a color-receptive layer formed thereon. What is first required of this image-receiving sheet is high sensitivity in printing and heat resistance. When the heat resistance is poor, heating at the time of printing causes curling or traces of a thermal head on the surface of the image-receiving sheet, deteriorating the image quality. Regarding the sensitivity in printing, an increase in a dye sublimation transfer recording speed in recent years has led to a strong demand for an image-receiving sheet having high sensitivity in printing.
The properties of the color-receptive layer are, of course, important to the sensitivity of the image-receiving sheet in printing. In addition, the properties of the substrate are also very important.
Various substrates have hitherto been proposed for the purpose of improving the sensitivity in printing and the heat resistance of the image-receiving sheet.
For example, Japanese Patent Laid-Open No. 136783/1989 teaches that a substrate which uses, as a part or the entirety thereof, a film having microvoids in its interior, prepared by extruding and biaxially stretching a resin composition comprising a mixture of polyethylene terephthalate with an inorganic pigment and an olefin, and which has a particular degree of cushioning, possesses high sensitivity in printing and thus can provide a sharp image.
Japanese Patent Laid-Open No. 168493/1989 teaches that good results can be obtained when a substrate prepared in the same manner as the substrate described in Japanese Patent Laid-Open No. 136783/1989 has closed cells in its interior and a particular specific gravity.
Japanese Patent Laid-Open No. 207694/1991 specifies the density of the substrate.
Japanese Patent Laid-Open Nos. 16539/1993 and 169865/1993 describe substrates having a particular percentage void, and Japanese Patent Laid-Open No. 246153/1993 describes a substrate comprising a particular material and having particular density and voids.
Further, Japanese Patent Laid-Open Nos. 115687/1989, 263691/1990, and 290790/1988 disclose substrates wherein the sensitivity in printing is improved by improving the cushioning and insulating properties.
According to the studies by the present inventors, however, all the above substrates are still unsatisfactory in at least one of sensitivity in printing and heat resistance.
Regarding properties required of the thermal transfer image-receiving sheet, in addition to the above described high sensitivity in printing and heat resistance, there is also an ever-increasing demand in the market in recent years for sufficient whiteness, opacity, and uniform appearance (uniform surface independently of whether the surface is glossy or matte), according to intended uses of image-receiving sheets.
Further, with a recent increase in recording speed (line speed) in the dye sublimation transfer system, the temperature of the thermal head of a printer is becoming higher. With an increase in the temperature of the thermal head, delamination between the substrate of the thermal transfer image-receiving sheet and the layers overlying the substrate is more likely to occur.
Especially in the case of an image-receiving sheet provided with a white opaque layer between the substrate and the colorant-receptive layer, since a white inorganic pigment is present in the white opaque layer, the adhesion between the substrate and the white opaque layer is likely to be poor, which is likely to cause delamination between the substrate and the white opaque layer during printing, making it impossible to provide a high-quality image. Further, the delamination gives rise to carrying error in a printer.
Various attempts have been made to enhance the adhesion between the substrate of the image-receiving sheet and a layer overlying the substrate.
For example, Japanese Patent Laid-Open No. 211089/1991 teaches a surface modification of a polyester film as a substrate by a corona or plasma treatment. However, the adhesive property imparted by the corona or plasma treatment is unstable and it decreases with the elapse of time.
Furthermore, Japanese Patent Laid-Open No. 211089/1991 describes an alternative method wherein a resin, such as an acrylic resin, having good adhesion both to the colorant-receptive layer and to the substrate is applied. However, the use as an adhesive layer of such resins as an acrylic resin, which are soluble in organic solvents, has the following problem. When a coating solution for a colorant-receptive layer, in which an organic solvent is generally used, is coated on the adhesive resin layer, the adhesive layer is attacked by the organic solvent contained in the coating solution, which remarkably deteriorates the appearance of the image-receiving sheet to lower the commercial value of the product.
Accordingly, an object of the present invention is to provide a thermal transfer image-receiving sheet having high sensitivity in printing and heat resistance.
Another object of the present invention is to provide a thermal transfer image-receiving sheet having a white opaque layer, which is excellent in adhesion between the substrate and the white opaque layer and has excellent appearance.